Order and disorder in (E)-[1-(biphenyl-4-yl)ethylidene]hydrazine: a structural, spectroscopic and theoretical study

An unexpected global disorder (co-existing rotational disorder and glide disorder) has been observed during an X-ray investigation of the crystal structure of (E)-[1-(biphenyl-4-yl)ethylidene]hydrazine, C₁₄H₁₄N₂, at room temperature. When the temperature decreases to 273 K, the disorder disappears, but the quality of the data set is low. The diffraction data were collected again at 110 K. Differential scanning calorimetry (DSC) analysis and polarizing-microscopy experiments, as well as a fourth set of single-crystal data collected at 283 K, proved that the order-disorder transformation occurs continuously. The analyses of these crystal structures and full-range relaxed potential energy surface scans showed that this kind of global disorder is not very difficult to achieve inside the crystal. Experimental and theoretical studies via UV-Vis and fluorescence spectra impart an understanding on the prediction methods of optical properties, which are essential for the rational design of biphenyl-based materials with pre-defined properties.